Conventionally, internal combustion engines use “spark ignition” (“SI”) to ignite a fuel-air mixture in the engine cylinder using an electrical spark. A further operating mode is also known, referred to as controlled auto-ignition (“CAI”), for example as described in Allen J., and Law D.,: “Variable Valve Actuated Controlled Auto-Ignition; Speed Load Maps and Strategic Regimes of Operation”, SAE paper no. 2002-01-0422. In CAI, hot exhaust gases from combustion are either retained in the engine cylinder or rebreathed from the exhaust manifold, so-called exhaust gas recirculation (“EGR”) for example by controlling the cylinder valves such that the exhaust valve is closed early or re-opened during the inlet stroke of the engine cycle to increase the trapped burnt gases. The mix of fuel, air and burnt gases self-ignites on compression. CAI operation has been shown to provide higher fuel economy and reduced pollutant formation compared with SI operation.
CAI is however only available over certain operating regimes of the engine. Outside these regimes, insufficient burst gas temperatures or the onset and increase of engine knock, which is associated with unwanted engine noise and can ultimately result in engine damage, becomes unacceptable and a transition to spark ignition operation is required.
An aim of the invention is to reduce or overcome the above problem.